Microfluidic devices are designed to precisely control fluid flows within geometrically constrained networks for a number of applications. In some examples, microfluidic devices may be employed to perform certain molecular diagnostic assays, such as those based on isothermal nucleic acid amplification methods (e.g., Recombinase Polymerase Amplification (RPA) or Nicking and Extension Amplification Reaction (NEAR)) to detect trace levels of nucleic acids. In some instances, microfluidic devices can facilitate point-of-care (POC) testing and can increase accessibility and speed of a diagnostic assay, such as an assay that can detect influenza (Flu) and Respiratory Syncytial Virus (RSV). For example, microfluidic devices may facilitate rapid detection of target nucleic acids present in Flu and/or RSV viruses.